1. Field of the Invention:
The present invention relates to maintaining the structure of viscous materials applied to semiconductor components. More particularly, the present invention relates to glob top application wherein viscous encapsulant material is prevented from flowing when applied to semiconductor components by a layer of de-wetting agent.
2. State of the Art:
Higher performance, lower cost, increased miniaturization of components, and greater packaging density of integrated circuits are goals of the computer industry. As components become smaller and smaller, tolerances for all semiconductor structures (circuitry traces, printed circuit board and flip chip bumps, adhesive structures for lead attachment, encapsulation structures, and the like) become more and more stringent. However, because of the characteristics of the materials (generally viscous materials) used in forming the semiconductor structures, it is becoming difficult to form smaller circuitry traces, conductive polymer bumps with closer pitches, adequate adhesive structures for leads attachment, and adequate encapsulation structures.
Material flow problems exist in the application of encapsulation materials. After a semiconductor device is attached to a printed circuit board ("PCB") by any known chip-on-board ("COB") technique, the semiconductor device is usually encapsulated with a viscous liquid or gel insulative material (e.g., silicones, polyimides, epoxies, plastics, and the like). This encapsulation (depending on its formulation) allows the semiconductor device to better withstand exposure to a wide variety of environmental conditions such as moisture, ion impingements, heat, and abrasion.
One technique used in the industry is illustrated in FIGS. 7-9. A stencil 50 is placed on a conductor-carrying substrate such as a PCB 52 such that an open area 54 in the stencil 50 exposes a semiconductor device 56 to be encapsulated as well as a portion of the substrate 52 surrounding the semiconductor device 56, as shown in FIG. 7. An encapsulant material 58 is then extruded from a nozzle 60 into the open area 54, as shown in FIG. 8. However, when the stencil 50 is removed, the encapsulant material 58 sags or flows laterally under the force of gravity, as shown in FIG. 9. This flowing thins the encapsulant material 58 on the top surface 62 of the semiconductor device 56, which may result in inadequate protection for the semiconductor device 56, particularly in the vicinity of the upper peripheral edges 64. Using a thicker encapsulant material would help minimize the amount of flow; however, thicker encapsulant materials are difficult to extrude through a nozzle and are subject to the formation of voids such as air pockets. These voids can cause delamination of the encapsulant from the PCB 52 or the semiconductor device 56, and if the voids contain moisture, during subsequent processing steps the encapsulant material may be heated to the point at which the moisture, usually in condensed form, vaporizes. Vaporization causes what is known as a "popcorn effect" (i.e., a small explosion) which at least damages (i.e., cracks) the encapsulation material and more often results in contamination of the device and usually irreparable damage thereto, effectively destroying the semiconductor device as a usable assembly.
In an effort to cope with the encapsulant flow problem, the damming technique shown in FIGS. 10-12 has been used. A high viscosity material 66 is extruded through a nozzle 68 directly onto a PCB or other carrier substrate 70 to form a dam 72 around a semiconductor device 74, as shown in FIG. 10, or a stencil can be placed on the substrate 70 such that a continuous aperture in the stencil exposes an area around the semiconductor device 74 to be dammed. The high viscosity material 66 is then disposed into the stencil aperture to form the dam 72. A low viscosity encapsulation material 76 is then flowed into the area bounded by the dam 72 by a second nozzle 78 such as a syringe or spray applicator, as shown in FIG. 11. The dam 72 prevents the low viscosity encapsulation material 76 from flowing and defines the periphery of the dammed encapsulated structure 80, shown in FIG. 12, after curing of material 76. The dam 72 can be made with high viscosity material without adverse consequences since it does not directly contact the semiconductor device 74 or form any part, other than a damming function, of the encapsulation of the semiconductor device 74. Although this damming technique is an effective means of containing the low viscosity encapsulation material 76, it requires somewhat duplicative processing steps as well as additional material and equipment, all of which increase the cost of the fabricated component.
Thus, it can be appreciated that it would be advantageous to develop a technique to control viscous material flow in the formation of semiconductor components while using commercially-available, widely-practiced semiconductor device fabrication techniques.